Calculation of shallow foundations

Calculation of shallow foundations is needed to clarify its geometrical dimensions and the choice of foundation species.It amounts to the calculation of three factors: the value of building pressure on the ground, the ground forces of pressure as a result of frost heaving and determining the strength of the foundation frame.

load of the building - a set of transmitted loads, simplified calculations - mass of the entire structure, distributed on 1 m2 of the bottom of the foundation plane.The strength of the swelling deformation is determined by the reference data for the particular soil type.The strength of the frame depends on the geometry of the basement and used fittings.

calculation load of the building

In order to calculate the mass of the building, it is necessary to have a project that takes into account the structure of dimensions, materials, construction, and other of its features.Calculation load on the foundation is carried out taking into account the winter snow load.The method of calculation is simple: calculate the weight of individual structural parts of the building, summarize them and divided by the foot of the basement area.Determine for this type of soil resistivity calculated soil resistance R0 and compares it to the received value of the specific load N. If R0 & lt;N, revise foundation shape or dimensions: it operates with a broad sole downward extension or increase its width.

frost heave and the choice of the type of foundation melkozaglublennogo

concept of "heaving soils" many baffled.Try to understand what it is.Different soils have different ability to accumulate moisture.Coarse sand, rocks do not retain water and clay, on the contrary, bind her, become plastic and long stay wet.Water expands during freezing, wherein the soil contains a lot of moisture is increased in size.This phenomenon is called "frost heave".

Depending on the composition and size of the soil particles can be divided into 5 groups - they are listed in Table 1.

Depending on the soil structure group selected melkozaglublennogo foundation in drawing and determine its overallsize, height and depth of the bedding.Calculate the area of ​​the lower base of the foundation Af.

method of calculating the deformation

calculation is carried outon two conditions:

• calculated deformation amount of swelling is not more than the allowable limit deformation;
• relative ground deformation, taking into account the load does not exceed the limit relative to a particular type of structure.

limit for a particular type of deformation of the structure is determined by Table 2.

To determine the value of these strains for a particular structure, you need to make a series of complex calculations.

swelling deformation calculated by the formula

deformation In this formula N - specific pressure of the entire structure on the ground, it is calculated by a separate procedure expressed in tons per 1 m2.

coefficient b depends on the ratio of the thickness of the bedding to the width of the base, it is determined by the table 3.

The value of Pr - on the sole of the heaving soils, for strip foundation is calculated as follows:

indicator b - width of the tape foundation, and ss - resistance of the frozen soil, it can be found in the SNIP 2.02.01-83.

power ground layer exposed buckle under the foundation is defined as dz dz = df - d - Hth where the df - the average depth of frost penetration, as determined by Table 4, and the values ​​of d and Hth - the height of the foundation and bedding thickness in meters.

Table 4 - Average depth of soil freezing by regions

After calculating the power ground dz layer is determined by the schedules coefficient of working conditions the freezing soil ka defined by schedules depending on the value dz and area valuesAf foundation base per unit length.

deformation swelling unloaded hfi base are according to the formula in Table 5, corresponding to the selected type of foundation and its geometric dimensions: depth of laying the foundation of d and Hth cushion thickness.

Table 5 - Calculation formulas for different types of soils

1. 1. determines the power the freezing layer of soil heaving dz = df - d - Hth.Estimated depth of freezing df for Moscow on Table 4 is 1.4 m. Dz = 1,4-0,7-0,5 = 0,2 m.
   2. foundation Determine the specific area per meter of its length, with the width of the basement 0,4m area is 0.4 m2.
   3. By drawing determine the coefficient ka, it is equal to 0.56.
   4. find the SNIP 2.02.01-83 rate σs - 64.
   5. determined by the formula t / m2.
   6. is given by m
   7. find the coefficient b from Table 3 for a belt-type foundation: the selected ratio of thickness to width of the bedding grounds of 0.5 / 0.4 = 1.25, he is the foundation of 0.88.
   8. load of the building, according to the calculations is equal to 23 t / m2.
   9. Define m = 0.1 cm.

   Permitted swelling deformation on the table 2 is 2.5 cm. The condition is met.

relative heave deformation structure, taking into account the stiffness of the frame is given by

Indicator w, being based on the flexibility of the structure factor designs l on VSN 29-85, determine theon the graphs.

Dhfp- difference of swelling deformation at the maximum and minimum pre-winter soil moisture.

L - the length of the structure of the wall, m

1. • determined by the method VSN 29-85 l value of the index of flexibility of the structure designs -. 0.55.
   • Determine schedule index value w - 0.03.
   • determined by the difference of swelling strains by the method of BCH 29-85.Dhfp = 0,0022 m
   • length of the structure of the walls is 10 and 8 m
   • Relative deformation taking into account the stiffness of the frame for long wall
   • m To m short wall
   • allowable value Table 2 -... 0.0005 m.The condition is met.

If the result of the calculation would be that the conditions are not met, it is necessary to increase the thickness of the cushion or the estimated area of ​​the basement, changing the width of the base.